1. Field of the Invention
The present invention relates to multilayer ceramic electronic components and methods for making the multilayer ceramic electronic components. In particular, the present invention relates to an external terminal electrode for a multilayer ceramic electronic component and a method for making the external terminal electrode.
2. Description of the Related Art
The market for small portable electronic appliances such as cellular phones, laptop computers, digital cameras, and digital audio equipment has grown in recent years. These portable electronic appliances not only continue to be miniaturized but also continue to provide higher performance. While portable electronic appliances include numerous multilayer ceramic electronic components, size reduction and higher performance are also demanded for the multilayer ceramic electronic components. For example, multilayer ceramic capacitors are required to be further reduced in size and to have a higher capacitance.
In order to reduce the size of multilayer ceramic capacitors and increase their capacitance, it is effective to reduce the thickness of ceramic layers. Recently, capacitors having ceramic layer thicknesses of 3 μm or less have been used in practical applications. Although further thickness reduction is currently being pursued, a problem arises in that short-circuiting between internal electrodes is more likely to occur when the thickness of the ceramic layers is reduced, thereby making it difficult to ensure the quality of the capacitors.
Another effective way is to increase the effective area of internal electrodes. However, for mass production of multilayer ceramic capacitors, side margins between the internal electrodes and the side surfaces of the ceramic element bodies and end margins between the internal electrodes and the end surfaces of the ceramic element bodies must be maintained in order to compensate for misalignment of stacked ceramic green sheets and deviations in cutting. Thus, the degree to which the effective area of the internal electrodes can be increased is limited.
To increase the effective area of internal electrodes while maintaining the required margins, the area of the ceramic layers must be increased. However, the degree to which the area of the ceramic layers can be increased is limited due to the designated size standards. Furthermore, the thickness of the external terminal electrodes is also a limiting factor.
Typically, external terminal electrodes of multilayer ceramic capacitors are formed by baking conductive paste applied on end portions of ceramic bodies. The mainstream method for applying the conductive paste is to dip end portions of ceramic bodies into conductive paste contained in a paste vessel. According to this method, the thickness of the conductive paste applied at the center portion of each end surface of the ceramic bodies tends to be greater than that of other portions due to the viscosity of the conductive paste. Thus, because the external terminal electrodes have thick portions (e.g., portions with a thickness exceeding 30 μm), the areas of the ceramic layers must be decreased.
To overcome such a problem, a method for forming external terminal electrodes directly by plating has been proposed (see, for example, International Publication No. 2007/049456). According to this method, plating films are grown from exposed portions (nuclei) of internal electrodes exposed at end surfaces of ceramic bodies, and the exposed portions of adjacent internal electrodes become connected to each other as the plating films grow. Thin, flat external terminal electrodes can be formed by this method as compared to the existing method that uses conductive paste.
However, during plating, the plating solution is likely to enter the internal conductors inside the ceramic bodies. The plating solution includes water-soluble conductive components including a metal complexing agent. Thus, when a ceramic electronic component in such a state is used in a high-temperature high-humidity environment, the presence of moisture and energization cause electrochemical reactions in the internal conductor portions where the plating solution has penetrated, which may result in deterioration and separation of the ceramic layers and decreased reliability of ceramic electronic components.
One way to avoid such a problem caused by the plating solution is to conduct heat treatment at a high temperature of, for example, at least 500° C. after plating to remove the plating solution penetrated inside the ceramic body.
However, when nickel is used as the internal conductor material and copper is used as the underlayer plating film material and when high-temperature heat treatment is performed after the copper plating, the density of the underlayer plating film decreases. This results in the penetration of water and/or the plating solution during subsequent formation of upper layer plating films and has an adverse effect of decreasing the reliability. This is presumably because of interdiffusion between the nickel defining the internal conductors and copper defining the underlayer plating film, which leads to the formation of many voids in the underlayer plating film due to the Kirkendall effect.